The specific properties of glass make it a suitable substrate for carrying electroconductive layers in electric or semiconductor devices such as flat panel displays, electroluminescent panels, cathode ray tubes (CRTs), photovoltaic cells, etc. In addition to a high thermal and dimensional stability, glass has many other beneficial properties compared to plastic materials, e.g. the ease of recycling, excellent hardness and scratch resistance, high transparency, good resistance to chemicals such as organic solvents or reactive agents, low permeability of moisture and gases, and a very high glass transition temperature, enabling the use of high-temperature processes for applying an electroconductive layer. However, the main problems associated with the use of glass as a substrate in electric or semiconductor devices are its high specific weight, brittleness and low flexibility. The latter problems require that the coating of a functional layer on glass is typically carried out in a batch process (sheet by sheet), whereas the application of layers on a plastic support is generally performed as a continuous process, e.g. by using a web coater or continuous printing techniques such as screen or offset printing. It is well-known that the productivity and cost efficiency of a continuous (web) coating process is significantly higher than of a batch (sheet) coating process.
Some applications require electroconductive substrates which are characterised by a low weight and sufficient flexibility, e.g. liquid crystal displays in portable devices, electroconductive substrates having a curved geometry such as in car dashboards or batteries, and photovoltaic cells (often called solar cells) for use on roofs or as a power supply of satellites, etc. For these applications, plastic foils may be used as substrate for carrying electroconductive layers in spite of the many disadvantages compared to glass. The high permeability of oxygen and water through plastic substrates degrades the electroconductive layers rapidly. Some progress has been made on producing plastic foils with barrier layers to limit said permeability; however the lifetime of electric devices in which such conducting plastic foils are used is still limited and needs to be improved.
In addition, an inorganic conducting layer such as indium-tin oxide (ITO) is brittle and as a result, the electroconductivity of an ITO layer is susceptible to deterioration by simply bending the flexible plastic substrate. All these effects limit the lifetime of such plastic-based devices considerably. Other problems associated with inorganic electroconductive layers such as ITO are
the high cost associated with vacuum-deposition techniques such as sputtering which are required to apply the inorganic layer; PA1 the high surface roughness of the inorganic layer which makes it difficult to apply thereon thin layers such as electroluminescent polymer layers used in organic light-emitting displays or diodes (OLEDs) or photoconductive polymer layers in solar cells; PA1 ITO requires an annealing step at an elevated temperature which is not compatible with most plastic substrates; PA1 ITO may generate oxygen which attacks adjacent layers such as poly(p-phenylenevinylene) used in OLEDs. PA1 sputtered inorganic layers are often characterised by the presence of a large number of pinholes. PA1 S1/glass/S2/electroconductive-layer PA1 glass/S1/S2/electroconductive-layer PA1 S1/glass/electroconductive-layer/S2 PA1 glass/S1/electroconductive-layer/S2 PA1 a reflecting cathode, e.g. a low-work function metal layer such as evaporated Ca. PA1 an electroluminescent layer, e.g. PPV; other suitable electroluminescent compounds are described in e.g. "Organische Leuchtdioden", Chemie in unserer Zeit, 31. Jahrg. 1997, No.2, p.76-86. PA1 an hole-injection layer (in this embodiment: the organic electroconductive layer) PA1 an transparent anode (in this embodiment the ITO layer) PA1 a transparent substrate (in this embodiment: the laminate)
Organic conducting layers have been developed which are not characterised by the above disadvantages. The production and the use of electroconductive polymers are well known in the art. In DE-A-41 32 614 the production of film-forming, electroconductive polymers by anodic oxidation of pyrroles, thiophenes, furans or aromatic amines (or their derivatives) is effected with a sulphone compound present in the electrolyte solution. The preparation of electroconductive polythiophenes and polypyrroles is described in U.S. Pat. No. 5,254,648 and in U.S. Pat. No. 5,236,627 respectively. In EP-A-440 957 a method for preparing polythiophene in an aqueous environment and applying polythiophene from an aqueous solution has been described. Such a solution is up until now mostly used in photographic materials as disclosed in e.g. U.S. Pat. No. 5,312,681, U.S. Pat. No. 5,354,613 and U.S. Pat. No. 5,391,472. In EP-A-686 662 it has been disclosed that layers of polythiophene coated from an aqueous composition could be made with high conductivity.
Such organic electroconductive layers can be coated on glass for various applications. WO 98/01909 describes a light-emitting diode wherein an ITO layer has been overcoated with an organic polymer selected from polyfurans, polypyrroles, polyanilines, polythiophenes and polypyridines. A similar combination has been described in Philips Journal of Research, Vol.51, No.4, p518-524 (1998). DE-A-41 29 282 discloses heat protection windows obtained by coating a polythiophene on glass or plastic sheets and DE-A-42 26 757 describes a similar antistatic and heat protection layer on a laminated glass such as a car windshield. The glass layers in a car windshield have a typical thickness of 2.1 mm. The electric resistivity of such antistatic and heat protection coatings has a typical value of &gt;10.sup.5 .OMEGA./square, which is far too high to be suitable as an electroconductive layer. EP-A-669 205 describes a glass/plastic laminate wherein an electroconductive layer is applied between the glass and plastic sheets of the laminate for de-icing or thermally insulating safety glass in vehicles. Such material can neither be used as an electrode because the electroconductive layer is insulated between the glass and plastic layer.